Electro-optical modulator structure

ABSTRACT

The present invention discloses an ultra-compact optical modulator comprising at least one resonator on a semiconductor chip. The EO modulator modulates incoming light having a certain wavelength range and comprises a waveguide layer accommodating at least one resonator having a periodic complex refraction index distribution structure defining a periodic defect band-edge and a cladding layer; and at least one electrode; the waveguide layer, the cladding layer and the electrode forming a capacitor structure; such that when an external voltage is applied to the capacitor structure the free carrier concentration in the waveguide layer is controlled, enabling a modulation of the resonator&#39;s refractive index; wherein the periodic defect band-edge is selected to be within the wavelength range, enabling a slow-light propagation of the incoming light within the waveguide layer.

RELATED APPLICATIONS

This application claims the benefit under 35 USC 119(e) of U.S.Provisional Application Nos. 61/033,177 filed on Mar. 3, 2008, and61/040,987 filed on Mar. 31, 2008, both of which are incorporated hereinby reference in their entirety.

FIELD OF THE INVENTION

The present invention relates generally to an electro-optical modulatorstructure and to different applications using the electro-opticalmodulator.

BACKGROUND OF THE INVENTION

The following references are considered to be pertinent for the purposeof understanding the background of the present invention:

-   [1] G. Muller, M. Muller, A. Wicht, R. H. Rinkleff, and K. Danzmann,    “Optical resonator with steep internal dispersion,” Phys. Rev. A 56,    2385-2389 (1997).-   [2] M. Soljacic, E. Lidorikis, L. Vestergaard Hau, J. D.    Joannopoulus, “Enhancement of microcavities lifetime using highly    dispersive materials,” Phys. Review E. 71, 026602 (2005).-   [3] Damian Goldring, Uriel Levy and David Mendlovic, “Highly    dispersive micro-ring resonator based on one dimensional photonic    crystal waveguide—Design and analysis”—Optics Express, 15, 3156-3168    (2007)-   [4] R. A. Soref, B. R. Bennet, “electro-optical effects in silicon”,    IEEE J. of Q. Elec. QE-23, 123-129 (1987)-   [5] A Liu, R. Jones, L. Liao, D. Samara-Rubio, D. Rubin, O.    Cohen, R. Nicolaescu, M. Paniccia, “A high speed silicon optical    modulator based on metal-oxide semiconductor capacitor”, Nature 427,    615-618-   [6] S. Stepanov, S. Ruschin, “Modulation of light by light in    silicon-on-insulator waveguides” App. Phy. Lett. 83, 5151-5153    (2003).

An electro-optical (EO) modulator is an essential component in anyoptical communication system. An EO modulator is an optical device inwhich a signal-controlled element is used to modulate light using theelectro-optic effect. The phase, the frequency, the amplitude, or thedirection of the input light may be affected (modulated). The mainfeatures usually required for the EO modulator are high speed,sufficient modulation depth, low losses and, as with any other device,robustness and reliability. The EO modulator may be used for intra-chipcommunication, in which high speed, small volume, and CMOS processcompatibility are required.

Traditional modulators are based on free space optics technology using,for example, polarization rotation. As technology advanced, differenttypes of integrated EO modulators were demonstrated. Most of themodulators are based on either a Mach-Zehnder Interferometer (MZI)configuration, or on a resonant configuration (Fabry-Perot, Ring, etc).In both cases, an electrical signal is used to modulate the freecarriers' concentration in the semiconductor to obtain opticalmodulation. Usually, a semiconductor EO modulator changes the resonantproperty of a resonator corresponding to an operating wavelength bycontrolling free carriers of a semiconductor, so as to serve as anoptical switch and accordingly to enable rapid transmission of digitalsignals.

Optical resonators are devices having internal optical path lengths thatare much longer than their physical dimensions. Long optical pathlengths are produced by multiple reflections of optical rays on mirrorsurfaces.

Two fundamental types of optical resonators are the Fabry-Perot cavityand the ring resonator. The Fabry-Perot cavity comprises twospaced-apart parallel reflective planes. Resonance (constructiveinterference of the reflected light) occurs for specific wavelengths oflight reflected between the reflective planes, when a wave traveling inthe resonator undergoes a 2πN phase retardation, where N is a wholenumber. Thus, the transmission resonances are periodic across thespectrum (assuming non-dispersive medium). Instead of reflective planes,reflective gratings can also be used to achieve similar results.

Ring resonators establish resonances in a similar manner, but thedistance between the reflective planes is defined by the circumferenceof a circular waveguide rather than the separation between tworeflective planes. The potential applications for such resonatorsinclude filters, sensors, optical delay lines, and more.

An important characteristic of optical resonators is its Quality factor(Q-factor) that is inversely proportional to the photon lifetime in theresonator (in time domain). Different applications require differentQ-factor values; however, obtaining higher Q-factors for a resonator istypically of high interest. The overall Q factor of a resonator is givenby 1/Q=1/Qabs+1/Qrad+1/Qc where Qabs, Qrad, and Qc are the absorption,radiation and coupling quality factors, respectively. Each of theQ-factor terms can be effectively increased by using a highly dispersivematerial inside the resonator [1, 2].

As for the materials used for the fabrication of the EO modulators,high-index materials (n≈3.5), such as silicon, enable strong lightconfinement and the fabrication of dense optical circuits. Thehorizontal confinement of the light is generally obtained by differentpatterns that are introduced to the substrate, while the verticalconfinement is usually created by a low-index or highly reflectivelayer. For example, for Silicon-On-Insulator (SOI)-based devices, havinggenerally a silicon substrate (Si) layer, and a top cladding (SiO2)layer, the horizontal confinement is created by the difference betweenthe index of refraction of the air and of the SiO2 (n≈1.5).

The optical properties of the silicon are expressed via its complexrefractive index. In order to tune the optical characteristics of aSOI-based device, the silicon's refractive index has to be changed, byusing, for example, the electrically induced method.

The method for electrically inducing refractive index changes in siliconis based on the modulation of the free carriers' concentration. Freecarriers are generally electrically injected to a silicon substrateusing a PN junction or a MOS capacitor. The injected carriers produce achange in the silicon's refractive index due to three major effects: (i)free-carrier absorption, (ii) Burstein-Moss effect, and (iii) Coulombicinteraction with impurities.

An analytic approximation to refractive index change (according to theDrude Model) is given by [4]:

$\begin{matrix}{{\Delta\; n_{FC}} = {{- \frac{{\mathbb{e}}^{2}\lambda^{2}}{8\;\pi^{2}c^{2}ɛ_{0}n}}\left( {\frac{\Delta\; N_{e}}{m_{ce}^{*}} + \frac{\Delta\; N_{h}}{m_{ch}^{*}}} \right)}} & (1)\end{matrix}$where ΔN_(e), ΔN_(h) are the changes in the free electrons and holesconcentrations respectively, and M_(ce)*, M_(he)* are the effectivemasses of free electrons and holes respectively. The analyticapproximation indicates the phenomenological behavior of the silicon,however, for more accurate calculations, the empirical equation (2)experimentally derived in reference [5] for 1.55 μm wavelength is used:Δn _(FC)=−8.8×10⁻²² ΔN _(e)−8.5×10⁻¹⁸(ΔN _(h))^(0.8)  (2)The electrons and holes densities are in cm⁻³ units.

Performing analytical calculation of the concentration of free carriersthat are injected to a SOI-based device is rather difficult. Basically,the continuity equation is solved for the charge carriers, and thegeneration rate in case of illumination is calculated [6]. The majorobstacle in such a process is the surface recombination which has astrong influence on the charges distribution. Since surface conditionsvary from one device to the other, and depend greatly on fabricationprocesses, it is very difficult to predict the charges distribution. Inorder to get some approximate results, one may use previously obtainedresults or perform some preliminary experiments.

SUMMARY OF THE INVENTION

There is a need in the art to improve the performance of electro-optical(EO) modulators, by reducing their size, reducing fabrication costs,increasing modulation depth and speed, and reducing supply power. Thepresent invention combines all of these requirements by providing anultra-compact optical modulator comprising at least one resonator on asemiconductor chip. The modulator sizes are approximately 5 μm×2 μm. Theoptical modulator modulates the light by either passing through orreflecting the un-modulated light. The incoming light passes through theoptical modulator to the output port only if its frequency matches theresonance frequency of the resonator. By modulating the resonator'sresonance frequency, light modulation is produced.

Therefore, there is provided an EO modulator for modulating incominglight having a certain wavelength range comprising: a waveguide layeraccommodating at least one resonator having a periodic complexrefraction index distribution structure defining a periodic defectband-edge and a cladding layer; and at least one electrode; thewaveguide layer, the cladding layer and the electrode forming acapacitor structure; such that when an external voltage is applied tothe capacitor structure the free carrier concentration in the waveguidelayer is controlled, enabling a modulation of the resonator's refractiveindex; wherein the periodic defect band-edge is selected to be withinthe wavelength range, enabling a slow-light propagation of the incominglight within the waveguide layer.

According to another broad aspect of the present invention, there isalso provided an optical device comprising at least one resonator havinga periodic complex refraction index distribution structure formed by aperiodic pattern of defects incorporated inside the resonator anddefining a periodic defect band-edge. The resonator is configured foroperating with an optical signal having a wavelength range including theperiodic defect band-edge, enabling a slow-light propagation of theoptical signal within the device, resulting in the modal dispersionincreasing within the device, the narrowing of the spectral linewidth ofthe resonator, and the enhancement of the quality factor of theresonator.

The resonator may be configured as a closed-loop resonator or as amicro-ring resonator. The radius of the resonator may be adjusted tosatisfy resonance conditions.

In some embodiments, the periodic complex refraction index distributionstructure is configured as a 1D photonic crystal. The resonator may beoptically coupled to at least one input/output waveguide. At least aportion of the input/output waveguide may have a periodic complexrefraction index distribution structure. The portion of the input/outputwaveguide may be located outside the coupling region between theinput/output waveguide and the resonator.

In some embodiments, the modal dispersion increasing within theresonator enables a reduction in the Free Spectral Range (FSR) of theresonator; the resonator generates ultra sharp peaks yielding ultra-highsensitivity.

The period of the complex refraction index distribution structure isadjusted such that the distribution structure band-gap matches theoperation wavelength of the optical signal.

The resonator may be made in a silicon substrate. In particular, theresonator, as well as the whole circuit may be fabricated inside a toplayer of a Silicon-On-Insulator (SOI) wafer. The substrate may be anyother semiconductor substrate, providing a horizontal confinement of thelight. On top of the silicon layer, a silicon-oxide buffer layeroperating as an insulator is deposited and then covered by a conductingor semi-conducting electrode. The silicon layer in which the resonatorexists, the silicon-oxide buffer layer and the electrode yield a type ofMOS capacitor.

By applying a voltage difference between the capacitor's contacts, thefree carriers' concentration in the silicon can be modulated, modulatingthe resonator's refractive index. If the Quality factor of the resonator(i.e. the ratio of resonance frequency and bandwidth) is sufficientlylarge, even a relatively small modulation of the refractive index wouldyield a sufficiently large modulation of the resonator's resonancefrequency. Thus, the electrical signal applied at the capacitors'contacts is converted to optical modulation of the incoming light.

In another broad aspect of the present invention, a hybrid resonatorstructure is realized by incorporating a series of periodic defects(i.e. a periodic complex refraction index distribution structure) into astandard resonator (e.g. Micro-Ring Resonators—MRRs).

The addition of these periodic defects enables to control the dispersionwithin the resonator structure. When the wavelength range of operationapproaches the band-edge of the periodic structure, the modal dispersionis significantly increased. The increasing of the dispersion leads tonarrowing the spectral linewidth of the resonator. The periodic defectsgenerate a slow light phenomenon in which the propagation of the opticalsignal within the resonator is done at a very low group velocity (i.e.the velocity is at least tens of times slower than the speed of light ina vacuum). The slow light phenomenon occurs around the band-edgecorresponding to the periodic structure, increasing the Q-factor of theresonator and reducing its Free Spectral Range (FSR). The band-edge ofthe periodic structure is the region where no electromagnetic wave canexist, regardless of the wave vector.

It should be noted that conventional optical elements utilizing theproperties of periodic defects pattern (e.g. photonic crystals) aremanufactured on photonic crystal substrate (e.g. a two dimensionalcrystal structure). The resonator of the present invention is aresonator incorporating a periodic defect pattern. This enables asimpler production process, and more degrees of freedom in theadjustment of the periodic band-gap to the wavelength of the operation(i.e. less dependency on the wavelength of operation).

In some embodiments, the resonator of the present invention isconfigured and operable as a sensor. The sensor may be wavelengthselective. The sensor may be configured to identify chemical orbiological or any other material in a solid, gas or liquid phase nearthe sensor. An optical signal is directed toward a resonator or a set ofresonators and then directed outside the modulator. A flow lineenclosing, for example, biological material or a gas may be located inthe area surrounding the resonator of the present invention. Theresonator environmental change induces a variation in the refractiveindex within the resonator. Since the resonance frequency issignificantly affected by the sensor environment, the structure of thepresent invention provides a multipurpose sensor using spectral analysis(i.e. filtering). Such a sensor can be used for bio-chemicalapplications, temperature/pressure measurements, blood analysis,measuring amounts of different gases in the air etc.

The sensor may be configured and operable to sense laser conditions, forexample to sense temperature conditions affecting the laser operation.

Alternatively, by using a tunable laser, or a laser tuned to theresonance frequencies of the resonators, one can verify whether theresonators are on resonance or off resonance.

In some embodiments, the resonator produces a spectral response enablingboth low and high sensitivity wavelength selection resulting in a widedynamic range wavelength selection.

In some embodiments, the resonator produces a unique field distributionat the resonance wavelengths. In addition to the sensing of theexistence of a material located in the area surrounding the sensor, thefield distribution enables the sensing of the spatial locations of suchmaterial. Therefore, the sensor is configured and operable to identifyspatial distributions of materials in the area surrounding the sensor.The sensor may also be configured as an optical gyroscope.

There is also provided by the present invention, an EO modulator formodulating incoming light having a certain wavelength range comprising:a waveguide layer accommodating a resonator with embedded periodicdefects i.e. having a periodic complex refraction index distributionstructure defining a periodic defect band-edge and a cladding layer; andat least one electrode; the waveguide layer, the cladding layer and theelectrode forming a capacitor structure; such that when an externalvoltage is applied to the capacitor structure, the free carrierconcentration in the waveguide layer is controlled, enabling amodulation of the resonator's refractive index; wherein the electrode islocated on the cladding layer above the waveguide layer in a diagonaldirection respectively to the waveguide layer such that the electricalfield generated by the electrode and the optical mode of the lightwithin the waveguide layer are not overlapping, substantially reducingoptical losses in the modulator.

In some embodiments, the electrode being located on the cladding layerabove the waveguide layer is configured and operable to enhance thequality factor of the modulator.

An electrical field is applied to the optical waveguide layer between apair of electrodes. The pair of electrodes may form a symmetricalstructure. One electrode may be a gate electrode, the other electrodebeing a ground electrode located in a spaced-apart relationship with theresonator, providing a spacer between the ground electrode and theoptical field propagating in the waveguide layer.

In some embodiments, the modulator is optically connected to the outputof an optical coupler having one input port in which the light istransmitted, and a second input port in which a ground electrode islocated.

In the present invention, the electrodes made of conductors orsemi-conductor materials are specially designed (topographically) toprevent significant losses in the area of the resonator, avoiding highinsertion losses which might lead to very low modulation depth. Theelectrodes may be non-transparent electrodes, for example made ofpoly-Si, metals etc., and may also be transparent electrodes made ofseveral materials such as ZnO:Al, In₂O₃, IR-ITO and Carbon-Nanotubes(CNTs) sheets.

The modulator may be based on a Fabry-Perot configuration with tworeflecting surfaces. The reflecting surfaces may be configured as 1Dphotonic crystal reflectors.

In some embodiments, the modulator comprises at least one other couplingwaveguide optically coupled to the waveguide layer, enabling thecoupling of the incoming light from the coupling waveguide to themodulator. The waveguide layer may have a straight configuration. Thecoupling waveguide may have a curved folded strip configurationfacilitating the coupling of the incoming light.

In some embodiments, the modulator is configured and operable as anadd-drop filter.

The waveguide layer may comprise a PN junction configured to modulatethe refractive index in the waveguide layer upon application of externalvoltage.

In some embodiments, the waveguide layer comprises a periodic pattern ofdefects forming at least one of the following: a 1D photonic crystal ora 2D photonic crystal.

While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof have been shown by wayof example in the drawings and will herein be described in detail. Itshould be understood, however, that it is not intended to limit theinvention to the particular forms disclosed, but on the contrary, theintention is to cover all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to understand the invention and to see how it may beimplemented in practice, embodiments will now be described by way ofnon-limiting example only, with reference to the accompanying drawings,in which:

FIG. 1 a schematically illustrates a cross-section view of the layerstructure forming the electro-optical modulator of the presentinvention;

FIG. 1 b schematically illustrates an example of the resonator structureaccording to one embodiment of the present invention;

FIG. 1 c schematically illustrates another configuration of theresonator structure according to another embodiment of the presentinvention;

FIG. 2 a represents two types of periodic defects embedded into a stripwaveguide;

FIG. 2 b graphically illustrates the dispersion curve of the periodicstructure of FIG. 2 a;

FIG. 3 graphically illustrates resonator radius values satisfying theresonance conditions as a function of propagation constant;

FIG. 4 graphically illustrates a comparison between the frequencyresponse near resonance of standard resonators and resonators withembedded periodic defects;

FIG. 5 graphically illustrates the power flow in a resonator structurewith embedded periodic defects;

FIG. 6 graphically illustrates the spatial distribution of theelectrical field in the resonator with embedded periodic defects of thepresent invention at the first six resonances of the resonator;

FIG. 7 a schematically illustrates a top view of a modulator accordingto one embodiment of the present invention;

FIG. 7 b schematically illustrates a section scheme beneath the gateelectrode of the modulator of FIG. 1 a;

FIGS. 8 a-b illustrate the cross-section of the waveguides implementedin the modulator of the present invention with (b) and without (a) ahole in the top silicon layer;

FIGS. 9 a-b show the cross-section of the modulator in the area of thecavity that includes the gate electrode (a) the GND electrode (b);

FIG. 10 represents a GND electrode positioning scheme in case ofmetallic electrode;

FIG. 11 a shows a cross-section of the modulator associated with twocoupling curved folded waveguides; FIG. 11 b illustrates the samewherein the modulator is formed in a 2D photonic crystal;

FIG. 12 a shows a cross-section of the modulator associated with twocoupling curved folded waveguides in which a PN junction is used for themodulation of the refractive index in the resonator; and

FIG. 12 b illustrates the same as FIG. 12 a, wherein the modulator isformed in a 2D photonic crystal.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a novel high performance EO modulator formodulating incoming light having a certain wavelength range i.e. awavelength of operation. Reference is made to FIG. 1 a, generallyillustrating a cross-sectional view of the layer structure forming themodulator of the present invention. To facilitate understanding, thesame reference numbers are used for identifying components that arecommon in all the examples.

In this non-limiting example, the EO modulator structure 100 can befabricated on a SOI wafer with the following characteristics: 230 nmthick resonator (c-Si) layer 102, 3 μm thick buffer (SiO2) layer 103 anda ˜500 μm thick handle (c-Si) layer 105. The top silicon layer 102deposited on the SiO2 103 is recovered by another SiO2 buffer layer 104,which constitutes the cladding of a ridge waveguide (450 nm wide) thatguides light in and out of the SOI chip. The resonator is located on themodulator layer inside the ridge waveguide. On top of the resonator, agate electrode 108 separated from the silicon layer 102 by an oxidebuffer 104 is implemented.

The present invention provides a novel EO modulator having a waveguidelayer accommodating a resonator with embedded periodic defects. If aperiodic defect, such as a photonic crystal (PhC), is inserted into aresonator, the period may be configured so that the band-edge created bythe periodic defect is located at the wavelength of operation. Thefrequency response of a standard ring resonator is approximatelyperiodic and is composed of transmission peaks or dips (depending on theconfiguration). However, as explained in reference [3], the frequencyresponse of the resonator with embedded periodic defects comprises aband-gap where no peaks occur, then after crossing the band-edge, peaksappear. The spacing between the peaks determines the Free Spectral Rangeof the resonator with embedded periodic defects. The Free Spectral Rangeincreases as the wavelength drifts away from the band-edge, as well asthe peaks' width.

Reference is made to FIGS. 1 b-c illustrating the structure of such aresonator with embedded periodic defects, i.e. two highly dispersivemicro-ring configurations.

FIG. 1 b illustrates a notch filter configuration in which the resonatorstructure 110 is configured as a micro-ring resonator comprising aseries of periodic defects 112 embedded in the resonator 110.

In some embodiments, the resonator 110 is configured as a micro-ringresonator having a diameter of about 100 μm.

The notch filter configuration includes a single coupling waveguide 114and a micro-ring resonator 110 with embedded periodic defects.

The input signal is fed through port I via a light source assembly 120,and is transferred to the resonator 110. The input signal may include aplurality of signals of different wavelengths transmitted through awavelength-division multiplexing (WDM). The signal is then outputtedthrough port II to a detector 122. In the present example, the lightsource assembly 120 and the detector 122 are located outside theresonator 110 appropriately optically coupled thereto. Thisconfiguration enables transmission from one port and transmission of anoutput signal from another.

In some embodiments, to enhance the power performance of the notchfilter, at least a portion of the coupling waveguide 114 also comprisesperiodic defects. The periodic defects should have a different period tothose of the micro-ring. These periodic defects may be configured asreflecting structures generally designated to prevent light from beingcoupled to unwanted ports.

FIG. 1 c illustrates an add-drop configuration including two couplingwaveguides 114 and 116, and a micro-ring 110 with embedded periodicdefects. The input signal is fed through port I, and is transferred tothe resonator 110. The signal is then outputted at resonance frequenciesthrough port IV.

As mentioned above, reflecting structures may also be added on thecoupling waveguides at ports II and IV to ensure maximization of powertransfer between ports I and III at resonance frequencies. Thisconfiguration enables high transmission efficiency.

It should be noted that the structure period of the reflectors isdifferent from the period inside the ring. The period inside the ringyields a slow light propagating mode used as described above while theperiod at the waveguides' ports prevents any propagation of modes, i.e.,acts as a reflector.

Different types of periodic defects can be introduced inside theresonator structure. It should be noted that although the differenttypes of periodic defects have different distribution of the dielectricstructures, they essentially yield similar dispersion curves. The maindifference may occur in propagation losses and fabrication complexity.

Reference is made to FIG. 2 a representing a typical circular holedefect embedded in a waveguide structure as well as a side corrugationdefect type. FIG. 2 b illustrates the dispersion curve of the resonatorof FIG. 2 a. The inset depicts the mode profile (Hy field—in planepolarization) of the waveguide's lowest mode. The dispersion curve iscalculated using conventional numerical methods. For calculations, thewaveguide's core refractive index was 2.798 (equivalent to the effectiverefractive index of a 240 nm thick Silicon Slab for in-planepolarization), the clad refractive index was 1, the core width was 450nm and the defect-hole diameter 180 nm. These parameters simulatetypical strip silicon waveguides. As can be seen from FIG. 2 b, severalband-gap regions are formed due to the periodicity and the strongdielectric contrast. More importantly, at the wavelength range, close tothe mode band edge, the dispersion curve is flattened and a highlydispersive mode is obtained.

It should be noted that the resonator's radius is large enough, suchthat the mode's shift due to the bend is small, and the mode can beassumed to be identical to that of a straight waveguide. This assumptionis particularly true for silicon waveguides, where the high refractiveindex of the core leads to strong mode confinement.

The effects of interest occur in the wavelength range where a resonanceexists, and also in resonance frequencies within a specific range ofpropagation constant values, towards the band-edge. Taking into accountthese two parallel requirements, the period of the structure has to beadjusted such that the band-gap matches the desired operation wavelength(e.g. 1.55 μm for communication applications).

In addition, the resonator perimeter has to make two conditions (i)phase matching, given by:2πR·KZ=m·2π  (1)where R is the ring's radius, KZ is the mode's propagation constant andm is an integer, and (ii) periodicity. For the Bloch modes to exist, aninteger number of periods within the resonator is required, i.e.2πR=q·P  (2)where P is the period and q is an integer. Equations (1) and (2) imposediscrete values of R for given values of KZ and P.

FIG. 3 depicts the minimum possible values of R satisfying resonanceconditions as a function of propagation constant (for various values ofKZ and a chosen P). It should be noted that according to the results inFIG. 3 a resonator radius of 13 μm (i.e. micro-ring resonator) issufficient to obtain a group index value of over 50. Moreover, in such aradii values regime, there is an increased amount of KZ values thatyield minimum radius values, making the resonator design much simpler.For example, if the radius of the ring is low, any fabrication deviationwould impose a significant deviation of the resonance wavelength; whilewhen using larger radii values, the wavelength deviation would be muchsmaller since there are many solutions that satisfy the minimum radiuscondition in this region.

Unlike the case of a standard resonator where the resonance wavelengthsare almost equally spaced, a rapidly changing free-spectral-range (FSR)is observed when approaching the band-edge wavelength and a band-gapregion where no resonance exists. The shrinking FSR results from theincrease of the group index inside the resonator. A resonator withembedded periodic defects can produce ultra sharp peaks (e.g. smalllinewidth) yielding ultra-high sensitivity.

The following is a comparison between two pairs of standards andresonators with embedded periodic defects in an add-drop configuration.The resonators with embedded periodic defects are based on theconfiguration presented in FIG. 1 c. Both resonators have a 1D PhC witha period of 0.37 μm. The first resonator with embedded periodic defectsincludes 100 periods while the second includes 150 periods. Their radiiare 5.89 μm and 8.83 μm respectively. The waveguide-resonator separationis set to 400 μm in both resonators. This pair of resonators withembedded periodic defects was compared to a second pair of resonatorswith a standard add-drop configuration and parameters identical to thoseof the resonators with embedded periodic defects (Radius andwaveguide-resonator separation). For the comparison, the in-waveguidereflectors of the resonator with embedded periodic defects are located(see FIGS. 1 b-c) at a specific distance so that the resulted phase, θis equal to π/2. The latter ensures that the coupling decay ratemultiplier is equal to unity and thus the multiplier does not affect thespectral width of the resonator's response.

FIG. 4 presents the resulted transmission peaks. The transmission peaksfrom the two standard resonators are denoted R1 and R2, while thetransmission curves from the two resonators with embedded periodicdefects are denoted R1′ and R2′. The transmission from both resonatorswith embedded periodic defects is narrower compared with thetransmission obtained by the standard resonator. This narrowertransmission implies higher Q-factors for the resonators with embeddedperiodic defects. An even more interesting outcome of FIG. 4 is foundwhen analyzing the peak width. When increasing the resonator radius,several processes affect the photon decay rate in the resonator: (i)increase in coupling into the waveguide, correspondingly increasing thephoton decay rate; (ii) the resonator perimeter is increased, leading toa decrease in the photon decay rate; (iii) stronger dispersion due tocloser proximity to the band-edge, further decreasing the decay rate(this effect is true only for the resonator with embedded periodicdefects). Thus, when increasing the resonator radius, there iscompetition between two (standard MRR) or three (MRR with embeddedperiodic defects) factors that may lead eventually to either an increaseor a decrease in spectral linewidth. As illustrated in FIG. 4, forstandard MRR, the width is slightly increased with the increase of theMRR radius, while a much more significant decrease in peak width isobtained for the MRR with embedded periodic defects. Therefore, thethird factor, i.e., the increase in dispersion of the MRR with embeddedperiodic defects, is probably the major reason for the increase inQ-factor.

FIG. 5 depicts the power flow in the resonator with embedded periodicdefects of FIG. 1 c. The power flows into the system from the upperwaveguide and is transferred to the lower waveguide through theresonator. The incident optical field is fed into port I. As illustratedin the figure, in the lower waveguide the optical power propagatesequally towards the left and the right (port III and port IV).

Therefore, the increase of the resonator's quality factor has beendemonstrated by comparing the frequency response of standard and MRRswith embedded periodic defects with similar parameters. It should benoted that it can also be demonstrated by comparing the MRR's withembedded periodic defects peaks as the resonance wavelength gets closerto the band edge (see reference [3] incorporated herein by reference).

FIG. 6 depicts the spatial distribution of the electric field in the MRRwith embedded periodic defects of the present invention at the first sixresonances of the resonator (1^(st)—closest to the band-edge). Differentdistributions are observed for the different transmission peaks. Due tothe periodic nature of the medium in the resonator, the resonant modes'field distribution is not plain sinusoidal (as in regular resonators).The field distributions have an envelope function that is multipliedwith the sinusoidal function. The closest transmission peak to theband-gap obtains a single lobe envelope function. The second peakclosest to the band-gap has a two lobe function; the next peak has athree lobe function and so on. In most distributions, high field and lowfield regions are observed in the ring enabling spatial sensing since alow field region is less sensitive to external materials than a highfield region. This special field distribution enables performing spatialsensing in the vicinity of the resonator. Furthermore, the aboveproperty enables spectral coding for different materials, i.e., greatlyincreased sensing.

In some embodiments, the micro-ring resonator with embedded periodicdefects of the present invention with a radius larger than 10 μmproduces an order of magnitude increase in the Q-factor.

The highly dispersive MRR are useful for a large variety of applicationsranging from optical delay lines through various types of communicationsfilters to enhancing nonlinear effects to ultra sensitive sensors. Alsodue to the circular nature of the MRR with embedded periodic defects itmight be useful for Sagnac effect applications, such as opticalgyroscopes. The spatial nature of the field modes in the MRR withembedded periodic defects enables micro-scale spatial sensing andperforming spectral coding of spatially distributed substances. Inaddition, the MRR with embedded periodic defects can be used to improvethe quality of standard MRR EO modulators enabling either reducing thevoltage needed for modulation or reducing the resonator's size.

In some embodiments, the sensor is wavelength selective. Thewavelength-selective modulation property of the resonator can be usedfor building wavelength division multiplexing (WDM) interconnections.The small insertion loss of the resonator makes it possible to cascademultiple resonators along a single waveguide and modulate each WDMchannel independently. It should be noted that in this implementation,the resonant wavelength of each ring resonator has to be equal to thewavelength of the corresponding WDM channel.

Reference is made to FIG. 7 a illustrating a modulator 100 based on aFabry-Perot configuration with an embedded MOS capacitor. It should benoted that in the figure, the oxide buffer is disregarded forpresentation reasons. Within the waveguide 106, two reflectors forming aFabry-Perot resonator are located within the ridge SOI waveguide 106.The reflectors may be configured as a 1D photonic crystal reflector. Thereflectors may be formed by etching holes or other types of structuresinto the top silicon layer 102 (shown in FIG. 1 a).

Reference is made to FIG. 7 b, illustrating the electrode configurationin the modulator 100 of the present invention. On top of the resonator140, a gate electrode 108 separated from the silicon layer 102 by anoxide buffer 104 is implemented. This electrode 108 and a ground (GND)electrode 170 placed outside the modulator on the input (or output)waveguide form the MOS capacitor that produces the modulation inside theresonator 140. In order to deposit the oxide buffer 104, the wholeresonator 140 was covered with a SiO2 layer 104. The gate electrodes 108are located above the waveguide part of the modulator 100 in thediagonal direction as illustrated in FIG. 7 b, in order to produce arefractive index modulation in the resonator 140.

It should be noted that, since the index modulation induced by carriermodulation is rather weak, the distance between the gate electrode andthe waveguide must be as small as possible. On the other hand, theelectrode, regardless of its material, creates in-cavity losses(diffraction, insertion etc.) therefore, to prevent quality factordegradation, it is preferable to locate the electrode as far as possiblefrom the waveguide. Generally, a metal gate electrode region addssignificant optical losses if its distance to the Si waveguide is tooshort, since it would overlap significantly with the optical mode field.The present invention solves the above problem by configuring themodulator such that the electrodes yield a relatively strong electricalfield in the vicinity of the optical resonator and at the same time havea very small overlap with the optical mode yielding low optical lossesand high modulation depth for the modulator. The configuration includeshaving an electrode structure around the resonator so that the electrodestructure still performs its electrical role of inducing changes incarrier density in the top silicon layer. The novel structure has asmall spatial overlap with the optical mode leading to small opticallosses. Such small losses improve the modulator's quality. For example,for a given modulators' volume of about 0.125 μm³ Q-factors larger than10000 can be obtained. Small operation voltage (inferior to 2V) istherefore needed for the modulation. The electrical field created in thecenter of the electrode structure is large, yielding strong opticalmodulation (e.g. superior to 10 dB).

The modulator may be based on MOS capacitor in SOI chips. The resonatorof the modulator is not restricted to a specific type of resonator inSOI.

In some embodiments, the modulator's resonator may be configured as a 1Dphotonic crystal (PhC) cavity resonator, a ring resonator, or a 2D PhCcavity resonator etc.

Reference is made to FIGS. 8 a-b illustrating two differentconfigurations of the waveguide in the modulator. The cross-section oftwo waveguides is represented (a) without and (b) with a hole in the topsilicon layer 102. The configuration illustrated in the cross-section ofFIG. 8 a may be implemented inside the modulator and in standard stripwaveguides while the configuration illustrated in the cross-section ofFIG. 8 b may be implemented in the hole areas forming the reflectors.

Reference is made to FIG. 9 a illustrating a cross-section of themodulator in the area of the cavity (e.g. resonator) comprising the gateelectrode 108. The gate electrode 108 can be made of poly silicon ormetals to obtain good quality and low cost electrodes, however, iffurther reduction of the cavity losses is needed, then conductive andtransparent electrodes such as ZnO:Al, In2O3, IR-ITO andCarbon-Nanotubes (CNTs) sheets can be utilized. It should be noted thatthe transparency of the electrode allows light to pass directly from theelectrode through to the waveguide, reducing insertion losses. Thesematerials usually have low reflectance and losses and also exhibit lowrefractive index, and therefore are better than poly silicon having anindex of ˜3.4.

It should be noted that conventionally, the gate electrode is positionedjust above the waveguide inducing an overlap between the electric fieldgenerated by the gate electrode and the optical mode propagating insidethe waveguide.

As illustrated in FIG. 9 a, in this specific example, the gateelectrodes 108 are positioned in the diagonal direction above thesilicon top layer 102 to yield a relatively strong electrical field inthe vicinity of the optical resonator and at the same time have a verysmall overlap with optical mode yielding low optical losses and highmodulation depth for the modulator. The EO modulator may include onlyone gate electrode placed at an optimized location above the resonatorand having optimal shape to prevent the overlap of the optical mode. Theelectrode structure induces changes in carrier density in the topsilicon layer with a small spatial overlap with optical mode leading tosmall optical losses. The electrical field created in the center of theelectrode structure is large, yielding strong optical modulation.

This novel configuration enables using modulators with lower qualityfactor, and/or using smaller voltage for modulation and/or yieldingdeeper modulation. It should be noted that higher quality factorsignifies that the spectral shape of the transmission function of themodulator is sharper. The sharper the shape is, the smaller the changein refraction index is needed to shift the modulator from transmissionmode to blocking mode. Smaller change of refraction index requires lowervoltage of operation. On the other hand, if lower voltage of operationis less needed, a modulator having a lower quality factor can beutilized since the losses induced by the electrode(s) would not be toolarge.

Reference is made to FIG. 9 b illustrating the cross-section of themodulator in the area that includes the ground (GND) electrode 170. TheGND electrode 170 is connected to the waveguide 106 via the top siliconlayer 102 to enable carriers' movement inside the modulator 100. The GNDelectrode 170 is configured and operable to inject and extract thecarriers into and out of the resonator. Since the top silicon layer is asemi-conductor, the potential imposed by the GND electrode istransferred via the top silicon layer. In some embodiments, the GNDelectrode 170 is located at close proximity to the resonator but not inthe resonator itself to provide a spacer between the GND electrode andthe field propagating in the waveguide, thereby reducing losses in theoptical signal.

In some embodiments, to reduce optical losses induced by the GNDelectrode, an optical directional coupler may be used before the cavity(e.g. resonator), as illustrated in FIG. 10. Using this configuration,the GND electrode is electrically connected to the modulator, while theoptical signal does not pass through the GND electrode. The coupler isconfigured as a short optical transmission path to transfer the opticalsignal from one input port to the modulator, when the ground electrodeis placed on the second input port. In this way the optical losses arenot induced by the ground electrode, which is kept electrically isolatedfrom the modulator.

In some embodiments, FIG. 1 a illustrates an add-drop configurationincluding two coupling waveguides 502 and 504, and a longitudinalmodulator 500. The input signal is fed through port I, and istransferred to the modulator 500. The signal is then outputted atresonance frequencies through port IV. When light of the appropriatewavelength is coupled to the modulator 500 by the input waveguide 502,it builds up in intensity over multiple trips in the longitudinal cavitydue to constructive interference. It can then be picked up by a detectorwaveguide 504. Since only some wavelengths resonate in the resonatorcavity, it functions as a filter. In this configuration, two couplingcurved folded waveguides are accommodated adjacent to the modulator,facilitating light coupling in and out of the modulator. Thisconfiguration enables complete electrical isolation of GND electrodesfrom the modulator as detailed above in which the optical signal doesnot pass through the GND electrode, preventing optical losses. Moreover,the present invention provides a straight EO modulator optically coupledto at least one input/output waveguide. It should be noted that thisconfiguration enables to provide a compact modulator (about 0.25 μm³)having a large Free Spectral Range of more than 400 nm. Two reflectors506 forming a Fabry-Perot resonator are located within the ridgemodulator 500.

FIG. 11 b illustrates another configuration for the modulator in which a2D photonic crystal is used to form the resonator.

In some embodiments, a PN junction (instead of MOS capacitor) may beused for the modulation of the refractive index in the resonator, asillustrated in FIG. 12 a. By applying a voltage difference between themodulator's contacts 602, the free carriers' concentration in thewaveguide can be modulated, modulating the resonator's refractive index.FIG. 12 b illustrates a PN junction made in a 2D photonic crystal.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

1. An electro-optical modulator for modulating incoming light having acertain wavelength range comprising: a waveguide layer accommodating atleast one resonator having a periodic complex refraction indexdistribution structure defining a periodic defect band-gap andband-edge, a cladding layer, and at least one electrode; said waveguidelayer, said cladding layer and said electrode forming a capacitorstructure; such that when an external voltage is applied to saidcapacitor structure the free carrier concentration in the waveguidelayer is controlled, enabling a modulation of the resonator's refractiveindex; wherein said periodic defect band-edge is selected to be withinsaid wavelength range and a period of said periodic refraction indexdistribution structure is adjusted such that the periodic defectband-gap matches an operation wavelength of said incoming light therebyenabling a slow-light propagation of said incoming light within saidwaveguide layer.
 2. The modulator of claim 1, wherein said resonator isconfigured as a closed-loop resonator.
 3. The modulator of claim 2,wherein said resonator is configured as a micro-ring resonator.
 4. Themodulator of claim 2, wherein the radius of said resonator is adjustedto satisfy resonance conditions.
 5. The modulator of claim 1, whereinsaid periodic distribution structure is configured as a 1D photoniccrystal.
 6. The modulator of claim 1, wherein said resonator isoptically coupled to at least one input/output waveguide.
 7. Themodulator of claim 6, wherein at least a portion of said input/outputwaveguide has a periodic complex refraction index distributionstructure.
 8. The modulator of claim 7, wherein said portion is locatedoutside the coupling region between the input/output waveguide and theresonator.
 9. The modulator of claim 1, wherein the modal dispersionincreasing within said resonator enables a reduction in the FreeSpectral Range (FSR) of the resonator; said resonator generating ultrasharp peaks yielding ultra-high sensitivity.
 10. The modulator of claim1, wherein said resonator is made in a silicon substrate.
 11. Themodulator of claim 1, configured and operable as a sensor.
 12. Themodulator of claim 11, wherein said sensor is wavelength selective. 13.The modulator of claim 11, wherein said resonator generates a spectralresponse enabling both low and high sensitivity wavelength selectionresulting in a wide dynamic range wavelength selection.
 14. Themodulator of claim 11, wherein said sensor is configured and operable tosense at least one of the followings: laser conditions and temperatureconditions affecting the laser operation.
 15. The modulator of claim 11,wherein said sensor is configured and operable to identify at least oneof the followings: chemical or biological material in the areasurrounding the sensor, solid material in the area surrounding thesensor, and spatial distributions of materials in the area surroundingthe sensor.
 16. The modulator of claim 11, wherein said sensor isconfigured as an optical gyroscope.
 17. The modulator of claim 1,wherein said electrode is located on the cladding layer above thewaveguide layer in a diagonal direction respectively to said waveguidelayer; such that the electrical field generated by the electrode and theoptical mode of the light within the waveguide layer are notoverlapping, substantially reducing optical losses in the modulator. 18.The modulator of claim 1, wherein said electrode is configured andoperable to enhance the quality factor of the modulator.
 19. Themodulator of claim 1, comprising a pair of electrodes configured andoperable to apply an electrical field to the waveguide layer, whereinsaid pair of electrodes forms a symmetric structure.
 20. The modulatorof claim 19, wherein one electrode is a gate electrode and the secondone is a ground electrode being located in a spaced-apart relationshipwith the resonator, providing a spacer between the ground electrode andthe optical field propagating in said waveguide layer.
 21. The modulatorof claim 20, wherein said modulator is optically connected to the outputof an optical coupler having one input port in which the light istransmitted, and a second input port in which a ground electrode islocated.
 22. The modulator of claim 1, wherein said electrode is made ofconductor or semiconductor materials.
 23. The modulator of claim 1,wherein said electrode is made of transparent material.
 24. Themodulator of claim 23, wherein said electrode is made of ZnO:Al, In2O3,IR-ITO and Carbon-Nanotubes (CNTs) sheets.
 25. The modulator of claim 1,wherein said modulator is based on a Fabry-Perot configuration havingtwo reflecting surfaces.
 26. The modulator of claim 25, wherein saidreflecting surfaces are configured as 1D photonic crystal reflectors.27. The modulator of claim 1, comprising at least one other couplingwaveguide optically coupled to said waveguide layer, enabling thecoupling of said incoming light from said coupling waveguide to saidelectro-optical modulator.
 28. The modulator of claim 27, wherein saidelectro-optical modulator is configured and operable as an add-dropfilter.
 29. The modulator of claim 27, wherein said waveguide layercomprises a PN junction configured to modulate the refractive index inthe waveguide layer upon application of external voltage.
 30. Themodulator of claim 27, wherein said waveguide layer comprises a periodicpattern of defects forming at least one of the following: a 1D photoniccrystal or a 2D photonic crystal.
 31. An optical device to be used withan optical signal having a certain wavelength range; said devicecomprising at least one resonator having a periodic complex refractionindex distribution structure formed by a periodic pattern of defects anddefining a periodic defect band-gap and band-edge; wherein a period ofthe periodic pattern is adjusted such that the band-gap matches theoperation wavelength of said optical signal and said periodic defectband-edge is selected to be within said wavelength range, enabling aslow-light propagation of said optical signal within the device.
 32. Thedevice of claim 31, wherein said resonator is configured as aclosed-loop resonator.
 33. The device of claim 32, wherein saidresonator is configured as a micro-ring resonator.
 34. The device ofclaim 32, wherein the radius of said resonator is adjusted to satisfyresonance conditions.
 35. The device of claim 31, wherein said periodicdistribution structure is configured as a 1D photonic crystal.
 36. Thedevice of claim 31, wherein said resonator is optically coupled to atleast one input/output waveguide.
 37. The device of claim 36, wherein atleast a portion of said input/output waveguide has a periodic complexrefraction index distribution structure.
 38. The device of claim 37,wherein said portion is located outside the coupling region between theinput/output waveguide and the resonator.
 39. The device of claim 31,wherein said resonator enables an increase in modal dispersion withinsaid resonator, inducing a reduction in the Free Spectral Range (FSR) ofthe resonator; said resonator generating ultra sharp peaks yielding toultra-high sensitivity.
 40. The device of claim 31, wherein saidresonator is made in a silicon substrate.
 41. The device of claim 31,configured and operable as a sensor.
 42. The device of claim 41, whereinsaid sensor is wavelength selective.
 43. The device of claim 41, whereinsaid resonator generates a spectral response enabling both low and highsensitivity wavelength selection resulting in a wide dynamic rangewavelength selection.
 44. The device of claim 41, wherein said sensor isconfigured and operable to sense at least one of the followings: laserconditions, and temperature conditions affecting the laser operation.45. The device of claim 41, wherein said sensor is configured andoperable to identify at least one of the followings: chemical orbiological material in the surrounding area of the sensor, solidmaterial in the surrounding area of the sensor, and spatialdistributions of materials in the surrounding area of the sensor. 46.The device of claim 41, wherein said sensor is configured as an opticalgyroscope.